The present invention is directed to a method and apparatus for monitoring layer erosion in a dry-etching process. Such an apparatus has a first electrode that electrically contacts a substrate to be etched, and a second electrode that is located above the first electrode, both electrodes being situated inside a process chamber.
It is known to use emission spectrometers for monitoring layer erosion, whereby the light emission of atoms that are in the layer material to be etched is observed with the emission spectrometer. The disappearance of a signal from the emission spectrometer corresponds to the final point in time of the etching of the layer.
The use of optical reflectometers is also known for etching transparent layers, these optical reflectometers generating a signal that is periodic over time and that exhibits a uniform waveform as long as the etching process is still on-going. When the final point of the etching is reached, the signal of the reflectometer changes either to a constant value or to a periodic signal having a different frequency. The latter is the case when a further transparent layer is located under the layer to be etched, the etching rate for this further transparent layer or second layer having a different value than that of the layer to be etched first. When changing from the first to the second layer, the signal usually exhibits a discontinuity that can be electronically detected. In certain cases, however, this discontinuity can be extremely weak, so that a traditional electrical circuit is not able to unambiguously recognize this discontinuity.
Another situation that frequently occurs is when (for the purpose of structuring, a transparent mask, for example) an exposed and developed photoresist having defined gaps extends over the transparent layer that is to be etched. In this case, it is not only the layer to be etched that is eroded at the gaps in the mask, but it is also the mask parts that are eroded. The rates at which the layer and mask become thinner during the etching process usually differ from one another, so that the signal of the photometer represents the superimposition of two different interference systems. This means that the sum of two sine wave type signals having different frequencies and amplitudes occurs in a first approximation. This somewhat more complicated signal waveform likewise changes when the layer to be etched has been completely etched. The sine wave type signal of the mask then remains and a new signal of a further transparent layer laying under the layer that has been completely etched appears. Finding that point in time in this confusion of mutually superimposing, wave-shaped signals at which one of two subsignals disappears and a further one appears is not only an extremely difficult situation to resolve for the operator of an etching apparatus, but in many instances is simply impossible to resolve.